PositionViewer/Assets/Scripts/Components/DateTimePicker.cs

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class DateTimePicker : MonoBehaviour
{
    public Dropdown Day;
    public Dropdown Month;
    public Dropdown Year;

    public DateTime Date { 
        get {
            var date = new DateTime(int.Parse(Year.options[Year.value].text), int.Parse(Month.options[Month.value].text), int.Parse(Day.options[Day.value].text));
            return date; } 
        set {
            Days();
            //Year.value = Date.Year.;
            Month.value = Date.Month - 1;
            Day.value = Date.Day - 1;
            oldYear = Year.value;
            oldMonth = Month.value;
        } 
    }

    int oldYear = -1;
    int oldMonth = -1;

    // Start is called before the first frame update
    void Start()
    {
        DateNow();
    }

    // Update is called once per frame
    void Update()
    {
        if (oldYear == -1 || oldMonth == -1) 
            DateNow();
        if (oldYear != Year.value || oldMonth != Month.value)
        {
            Days();
            oldYear = Year.value; 
            oldMonth = Month.value;
        }

    }

    void Days()
    {
        Day.options.Clear();
        var year = int.Parse(Year.options[Year.value].text);
        var month = int.Parse(Month.options[Month.value].text);
        var days = DateTime.DaysInMonth(year, month);
        for (int d = 1; d <= days; d++)
            Day.options.Add(new Dropdown.OptionData($"{d}"));
        Day.value = 0;
    }

    void DateNow()
    {
        for (var y = 2020; y <= DateTime.Now.Year; y++)
            Year.options.Add(new Dropdown.OptionData($"{y}"));
        for (var m = 1; m <= 12; m++)
            Month.options.Add(new Dropdown.OptionData($"{m}"));       
        Year.value = Year.options.Count - 1;
        Month.value = DateTime.Now.Month - 1;       
        oldYear = Year.value;
        oldMonth = Month.value;
        Days(); 
        Day.value = DateTime.Now.Day - 1;
    }
}